The impact of bulk and surface defect states on the vibrational and optical properties of step-graded epitaxial GaAs₁₋ySby (0 ≤ y ≤ 1) materials with and without chemical surface treatment by (NH₄)₂S was investigated. Tunable antimony (Sb) composition GaAs₁₋ySby epitaxial layers, grown by solid source molecular beam epitaxy (MBE), were realized on GaAs and Si substrates by varying key growth parameters (e.g., Sb/Ga flux ratio, growth temperature). Raman and photoluminescence (PL) spectroscopic analysis of (NH₄)₂S-treated GaAs₁₋ySby epitaxial layers revealed composition-independent Raman spectral widths and enhanced PL intensity (1.3x) following (NH₄)₂S surface treatment, indicating bulk defect-minimal epitaxy and a reduction in the surface recombination velocity corresponding to reduced surface defect sites, respectively. Moreover, quantification of the luminescence recombination mechanisms across a range of measurement temperatures and excitation intensities (i.e., varying laser power) indicate the presence of free-electron to neutral acceptor pair or Sb-defect-related recombination pathways, with detectable bulk defect recombination discernible only in binary GaSb PL spectra. In addition, PL analysis of the short- and long-term thermodynamic stability of sulfur-treated GaAs₁₋ySby/Al₂O₃ heterointerfaces revealed an absence of quantifiable atomic interdiffusion or native oxide formation. Leveraging the combined Raman and PL analysis herein, the quality of the heteroepitaxial step-graded epitaxial GaAs₁₋ySby materials can be optimized for optical devices.